1. Field of the Invention
The invention relates to telecommunications, and more particularly, to transmission techniques that reduce bit error rates associated with analog clipping in discrete multitone-based transmission systems.
2. Description of the Related Art
Discrete multitone (DMT) is a standard modulation technique employed in various digital subscriber line (XDSL) communication systems. For example, asynchronous DSL (ADSL) systems commonly transmit data under a DMT modulation scheme. A DMT line code comprises multiple carriers or subchannels implemented through a Discrete Fourier Transform (DFT). Each subchannel is independently modulated to some carrier frequency. The number of subchannels available to carry information is generally equal to or slightly less than half of the DFT size. Each subchannel of a DMT line code employs a two-dimensional signal (e.g., a phase and amplitude modulated signal) equivalent to that of a passband single carrier system.
Quadrature amplitude modulation (QAM) is a modulation method that is used to encode a variable number of bits into such a two-dimensional signal. A number of bits are mapped into the in-phase and quadrature components of a complex symbol that is typically converted to an analog form, and then transmitted over the corresponding subchannel. The total average power of such a DMT-based system is the average power sum of each carrier as expressed as the sum of the average power of the ith carrier (PTi), from i=1 to m, where m is the number of carriers. Each of these carriers is associated with a crest factor. A crest factor is defined as the ratio of the peak to the root-mean-squared (RMS) level of the signal.
The crest factor for each carrier (the ratio of the carrier""s peak voltage to the RMS level of that carrier) is referred to as PARcarrier, for Peak-Average Ratio. In general, if the peaks of all carriers can be aligned, the peak voltage of a DMT system is equal to the peak voltage sum of all the carriers. Assuming that the average power of each carrier is the same, the crest factor of the DMT line code, referred to as PARDMT, can be calculated       PAR    DMT    =            m        *                  ∑                  i          =                      1            ⁢                          xe2x80x83                        ⁢            …            ⁢                          xe2x80x83                        ⁢            m                                    xe2x80x83                    ⁢              xe2x80x83            ⁢                        PAR                      carrier            ⁢                          xe2x80x83                        ⁢                          (              i              )                                      .            
Clipping of a DMT transmission signal where the peaks of all the carriers are aligned is statistically an expected occurrence. More specifically, signal clipping will occur with a probability determined by the Gaussian distribution of the DMT transmission signal. With this anticipated clipping in mind, the analog line driver circuitry of the transmission path is designed to limit the voltage swing of the line driver amplifier so that only the required peak power is delivered to the transmission line. The result is that the power dissipation in the line driver amplifier is limited to no more that is allowed for a given bit error rate. However, a clipped transmission signal at an analog stage is associated with various problems.
For example, when an analog line driver is driven into an overload condition thereby clipping the signal being transmitted, a recovery period is required as the internal bias circuitry of the driver re-adjusts to linear operation (assuming the input stimulus causing the overload condition is eliminated). Moreover, the transmission signal is distorted during this recovery period. Noise associated with this distortion causes performance degradation (e.g., increase in bit error rate), and inefficient use of the allocated transmission signal power.
One solution for dealing with the problems associated with transmission signal clipping is to increase the voltage swing of the analog line driver. Such an increase allows for an increase in the transmission signal crest factor, which in turn reduces the clipping probability as well as the bit error rate. However, increasing the voltage swing of the analog line driver also increases the total power consumption (e.g., power consumption in the analog line driver increases).
What is needed, therefore, is a technique that provides a solution to the problems associated with analog clipping in a DMT-based transmission system.
One embodiment of the present invention provides a method for conditioning a signal for transmission in a DMT-based communication system, the method comprising: converting the signal in a digital environment of the DMT-based communication system into a pre-clipped signal to avoid subsequent analog clipping in an analog environment of the DMT-based communication system. In one embodiment of this method, the signal is amplified until it has a shape that defines a linear performance envelope of the analog environment. In an alternative embodiment, the signal is amplified until it has a shape that corresponds to a maximum degree of anticipated analog clipping. The amplifying can be performed, for example, by a DSP process having a gain stage with a digital saturation point. In this embodiment, the digital environment can be associated with one of an overall unity gain or a less than unity overall gain.
Another embodiment of the present invention provides a system for conditioning a signal for transmission in a DMT-based transmission system, the system comprising: a digital environment configured to convert the signal into a pre-clipped signal to avoid subsequent analog clipping in an analog environment of the DMT-based communication system. In one embodiment of this system, the signal is amplified in the digital environment until it has a shape that defines a linear performance envelope of the analog environment. In another embodiment of this system, the signal is amplified in the digital environment until it has a shape that corresponds to a maximum degree of anticipated analog clipping. The digital environment can include, for example, a DSP process having a gain stage with a digital saturation point for amplifying the signal into digital saturation. In this embodiment, the digital environment can be associated with one of an overall unity gain or a less than unity overall gain.
The features and advantages described in the specification are not all inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.